As a rule, start-up elements for automatic transmissions are arranged in the drive train behind the engine and in front of the transmission, in order to accommodate the speed difference between the lowest operational speed of the engine and the stationary gearbox input shaft, which occurs upon the start-up of the vehicle from the stopped position. Force-locked clutches are typically used as start-up elements in manual transmissions, and hydrodynamic torque converters are employed with automatic transmissions.
The use of a brake at the sun gear of the incoming gear set of an automatic transmission, as a start-up element, is known from the publication “Getriebe in Fahrzeugen 2001” [“Transmissions in Vehicles 2001”] P. 461 p., Image 6, VDI-Berichte 1610 [VDI Reports 1610]. However, if a slip of the start-up element is required or desired in all gear selections, for example, for the purpose of an acoustic improvement, the brake at the sun gear described in this publication is not suitable as a start-up element, because it does not support torque transfer in its highest gear.
Wet start-up clutches have been recommended as start-up elements as well; however their structure is highly complex.